A research team, led by Silicon Valley technology firm Picarro, installed monitors in Davos and on a nearby mountainside to measure the carbon emissions tied to the annual gathering of high-powered government and corporate officials. But weeks before the WEF began Jan. 25, emissions were already 35% higher than expected, Picarro says.

The apparent surge in greenhouse-gas output wasn’t caused by preparations for the WEF, says Picarro chief executive Michael Woelk. Instead, it’s because the existing greenhouse-gas inventory for Davos is flawed and understates the city’s real carbon footprint, Woelk says. Other cities around the world are making similar mistakes, he says.

“None of them are actually measuring their emissions, they’re estimating their emissions,” says Woelk, whose company sells “analyzers” that monitor air pollution in real time. Errors in city-level emissions reporting could total as much as 4.5 billion metric tonnes of carbon dioxide a year, according to Picarro. That’s almost three times the carbon-dioxide output of Russia, according to International Energy Agency data.

“We’ve got to get down to what the real numbers are,” Woelk says. “The only way to do that is counting molecules in the air.”

Most greenhouse-gas inventories use a “bottom-up” approach to calculating the carbon footprint of a city, state, country or corporation. They collect data on coal, oil, natural gas and electricity use, and then estimate how many tons of carbon dioxide were released during the production and consumption of those energy sources. Once this “baseline” is established, the inventory is regularly updated to see whether emissions rise or fall over time.

It is very difficult to add up, and aggregate from the bottom up, what your emissions are – Davis

But some fossil-fuel consumption can be overlooked or double-counted, the source data can be wrong, and the estimates may rely on poor assumptions. Last year, researchers at the University of Texas and Carnegie Mellon University studied the greenhouse-gas inventories of 18 US cities and found “current inventory practices may lead to unrepresentative and thus inappropriate emissions baselines.” As a result, the study warns that carbon-reduction targets set by those cities “may be misguided.”

Real-time measurement of carbon concentrations in the air over cities can identify flaws in greenhouse-gas inventories and make sure they are fixed, says Ken Davis, a meteorology professor at Penn State University and a partner in the Davos project. “It is very difficult to add up, and aggregate from the bottom up, what your emissions are,” Davis says. With a network of emissions monitors, he says, a city can review its greenhouse-gas inventory and ask, “Does the atmosphere confirm this?”

Confounding Measurements

Carbon dioxide and other greenhouse gases are ultimately mixed evenly throughout the Earth’s atmosphere. But, at first, carbon emissions linger for as many as two days at lower altitudes near the smokestacks and tailpipes that released them, Davis says. That’s enough time to measure these low-level carbon concentrations and calculate how many tons of greenhouse gases were emitted locally, he says.

The “top-down, bottom-up” approach to greenhouse-gas inventories is being tested on a larger scale in Indianapolis, Davis says. The US Midwest city, almost 70 times bigger than Davos, is hosting a network of towers that continuously monitor low-level carbon emissions. The Indianapolis Flux Experiment, or INFLUX, will also monitor energy-consumption data and use aircraft to help measure carbon concentrations, according to a summary of the project. The National Institute of Standards and Technology is funding INFLUX, which includes researchers from Penn State, Purdue University, Arizona State University, the University of Colorado and the National Oceanic and Atmospheric Administration.